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Carboxylated dithiafulvenes and tetrathiafulvalenevinylogues: synthesis, electronic properties,and complexation with zinc ionsYunfei Wang and Yuming Zhao*
Full Research Paper Open Access
Address:Department of Chemistry, Memorial University, St. John’s, A1B 3X7,NL, Canada
Scheme 2: Synthesis of carboxylated TTFV 6 and DTF 7.
several decades ago, while the past few years have witnessed
surging research activities on integrating TTFVs into a variety
of π-conjugated molecular and macromolecular systems [12,17-
26]. In many of the studies, the remarkable redox activity and
intriguing conformational switching properties of TTFVs were
taken advantage of to enhance structural and electronic prop-
erties as well as to introduce some “intelligent” functions such
as conformational switchability and selectivity in terms of
molecular recognition and supramolecular interactions.
Scheme 1: Redox-induced conformational switching of diphenyl-TTFV1.
The synthesis of TTFV structures is usually carried out through
a facile iodine-promoted oxidative dimerization reaction of
corresponding dithiafulvene (DTF) precursors [27]. This
straightforward C–C bond forming reaction has not only
allowed TTFV derivatives with different substituents to be
readily assembled, but served as an effective methodology to
construct the π-conjugated frameworks of some TTFV-based
macrocycles and polymers [12,28]. Previously, we have investi-
gated a series of diphenyl-TTFVs with alkynyl groups attached
to the phenyl units as synthetic building blocks, through which
extension of π-conjugated structures could be conveniently
executed via the Pd-catalysed coupling and Cu-catalysed
alkyne–azide cycloaddition (i.e., click) reactions [22-26]. In this
work, we continued to explore the class of carboxylated
diphenyl-TTFVs, in view of the synthetic versatility of the
carboxyl group towards various commonly used linkage groups
(e.g., amides, esters). The carboxyl group also presents a reli-
able and useful ligand to coordinate with transition metal ions,
which in turn provides easy access to novel organic–inorganic
hybrid materials. The most notable example of research in this
context is the recent development of metal organic frameworks
(MOFs), wherein the design and synthesis of carboxyl func-
tional ligands has played a pivotal role prompting the advance-
ment of this field [29-31]. Very recently, some TTF-based
ligands have been employed to achieve organic–inorganic
hybrid materials with redox activity [32-35]; however, the use
of TTFVs as ligands has not been reported in the literature prior
to this work. This article thus describes the first exploration of
the synthesis and properties of a carboxylated diphenyl-TTFV 6
(Scheme 2) and its ability to form new redox-active porous ma-
terials through the formation of coordination polymer with
Zn(II) ions.
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959
Results and DiscussionThe synthesis of carboxyl-TTFV 6 and its DTF precursor 7 was
conducted via a route well established for aryl-substituted
TTFV derivatives [22,23]. As shown in Scheme 2, thione 2 was
first reacted with benzaldehyde 3 in the presence of
trimethylphosphite at 105 °C [36]. This olefination reaction
went completion within 3 hours to give DTF 4 in 77% yield
after column separation. Compound 4 was then subjected to an
oxidative dimerization in CH2Cl2 at room temperature using
iodine as oxidant. The dimerization gave TTFV 5 as a stable
yellow solid in 80% yield. Saponification was then performed
on compound 5 in a solution of NaOH in water and methanol to
finally afford carboxyl-TTFV 6 in 84% yield. Compound 6
showed relatively poor solubility in non-polar organic solvents,
but could be readily dissolved in polar solvents such as MeOH,
EtOH, THF, and DMSO. For comparison purposes, carboxyl-
DTF 7 was also prepared by hydrolysis of DTF 4 using similar
reaction conditions.
With carboxyl-TTFV 6 in hand, the preparation of coordination
polymers with Zn(II) ions was undertaken. As outlined in
Scheme 3, compound 6 was first mixed with two molar equiva-
lents of Zn(NO3)2·6H2O in EtOH, and to this solution triethyl-
amine was allowed to slowly diffuse in [37]. In a period of
4 days, coordination polymer 8 was gradually formed as a
yellow coloured crystalline solid, which was insoluble in
common solvents. By the same approach, complexes of
carboxyl-DTF 7 with Zn(II) ions were also produced as a
yellow powder.
The electronic properties of TTFVs 5 and 6 as well as their DTF
precursors 4 and 7 were investigated by UV–vis absorption
spectroscopy. Figure 1 shows the UV–vis absorption spectra of
these compounds, in which the maximum absorption wave-
lengths (λmax) of all the compounds appear to be nearly iden-
tical at ca. 385 nm. There are, however, slight variations in the
cut-off energies of long-wavelength absorption bands. The
origins of these long-wavelength absorption bands are mainly
due to HOMO to LUMO, HOMO to LUMO+1, and HOMO−1
to LUMO+1 transitions according to time-dependent density
functional theory (TD-DFT) calculations (see the Supporting
Information File 1 for details). The UV–vis data indicates that
the degrees of π-delocalization for the TTFVs and DTF com-
pounds are quite similar. This result is congruous with the fact
that diphenyl-TTFVs generally prefer a twisted cisoid con-
formation in the ground state [12,15,16], which in theory
significantly disrupts the π-delocalization within the molecules.
Therefore, even though the molecular sizes of TTFVs 5 and 6
are doubled in comparison with their DTF precursors 4 and 7,
the degrees of π-electron delocalization in these molecules are
still retained at a similar level in the ground state.
Scheme 3: Complexation of compounds 6 and 7 with Zn(II) ions.
Figure 1: UV–vis spectra of TTFVs 5 and 6 (solid lines) and DTF 4and 7 (dashed lines). Compounds 4 and 5 were measured in CH2Cl2,while compounds 6 and 7 were in THF.
The electrochemical redox properties of compounds 4–9 were
characterized by cyclic voltammetry, and the detailed cyclic
voltammograms are shown in Figure 2. For methyl ester-
appended DTF 4 (Figure 2A) an anodic peak was observed at
+0.82 V in the first cycle of scan, which is due to the single-
electron oxidation of the dithiole moiety into the dithiolium
radical cation [15,16]. In the reverse scan, a cathodic peak
emerged at +0.54 V which is assigned to the bielectronic reduc-
tion of the TTFV product electrochemically generated on the
Beilstein J. Org. Chem. 2015, 11, 957–965.
960
Figure 2: Cyclic voltammograms of compounds 4–9. Experimental conditions: supporting electrolyte: Bu4NBF4 (0.1 M), working electrode: glassycarbon, counter electrode: Pt wire, reference electrode: Ag/AgCl (3 M NaCl), scan rate: 200 mV s−1. Compounds 4 (1.5 mM), 5 (0.73 mM), 8, and 9were measured in CH2Cl2. Compounds 6 (0.76 mM) and 7 (1.5 mM) were measured in CH3CN.
electrode surface via the DTF dimerization reaction [15,16]. In
the following scan cycles, the redox wave pair characteristic of
TTFV at Epa = +0.62 V and Epc = +0.54 V was found to gradu-
ally increase in intensity as a result of increasing electrochem-
ical dimerization. The same electrochemical patterns can be
seen in the cyclic voltammograms of carboxyl-DTF 7 and
Zn-DTF complex 9 (Figure 2C and 2E); however, their redox
potentials showed a slight degree of variation. Experimentally,
the cyclic voltammogram of 9 was determined from its solid
thin film compressed on the working electrode surface. It is
interesting to note that Zn-DTF complex 9 retained the redox
activity and electrochemical reactivity of DTF even in the solid
state. The cyclic voltammograms of compounds 5 and 6 both
featured a reversible redox wave pair due to the simultaneous
bielectronic transfers occurring at the TTFV moieties
(Figure 2B and 2D). In the cyclic voltammogram of Zn-TTFV
coordination polymer 8 (measured from a solid film prepared in
the same way as 9), the redox wave pair of TTFV is discernible
but much weaker than that of Zn-DTF 9 (Figure 2F), suggesting
that the electrochemical activity of the coordination polymer is
considerably reduced in comparison with the smaller-sized
Zn-DTF complex.
The structural properties of Zn-TTFV coordination polymer 8
and Zn-DTF complex 9 were examined by IR spectroscopy
(Figure 3). Compared with the IR spectra of carboxyl-TTFV 6
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961
Figure 3: FTIR spectra of compounds 6–9.
and carboxyl-DTF 7, the vibrational bands of free carboxyl
groups were clearly absent in the spectra of 8 and 9, confirming
that the carboxyl groups were completely coordinated with
Zn(II) ions. The crystalline properties of coordination polymer
8 were examined by powder X-ray diffraction (PXRD) analysis.
The diffraction patterns shown in Figure 4 confirm that co-
ordinate polymer 8 possesses crystallinity in the solid state.
Actually, the diffraction patterns were found to bear resem-
blance to those of zincite ZnO. Such crystalline features hence
point to a possibility of the coordination polymer to take some
kind of framework-like structures in the solid state with certain
microscopic porosity.
To assess the porous properties of Zn-TTFV 8, nitrogen gas
adsorption analysis was conducted at 77 K. The adsorption
isotherm shown in Figure 5A indicates a Type-II adsorption
behaviour. Application of the Brunauer–Emmett–Teller (BET)
model gave a BET surface area of 148.2 m2 g−1 and an adsorp-
tion average pore diameter of 10.2 nm. The pore size distribu-
tion analysis data revealed that Zn-TTFV coordination polymer
8 carried microporosity primarily in the range of tens of
nanometers. Scanning electron microscopic (SEM) imaging was
performed on the particles of 8 to show some kind of crys-
talline-like micromorphology. In line with the gas adsorption
results, there were no relatively large pores on the micron scale
observable in the particles (see the inset of Figure 5B). On the
submicron scale, however, corrugated microporous features
could be clearly observed (see Figure 5B). To understand the
origin of the micropores, nitrogen gas adsorption experiments
were performed on Zn-DTF complex 9. The experimental
Figure 4: PXRD patterns of the Zn-TTFV coordination polymer 8 (redline) in comparison with the diffraction data of zincite ZnO (blue bars).
Figure 5: (A) Nitrogen adsorption isotherm of coordination polymer 8measured at 77 K. Inset: pore size distributions. (B) SEM image of thepowder of 8.
Beilstein J. Org. Chem. 2015, 11, 957–965.
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Figure 6: DSC traces of compounds 6–9 measured under a nitrogen atmosphere. Scan range: 25–400 °C, scan rate: 10 °C min−1.
results did not lead to any meaningful measurements of BET
surface area and microporosity, indicating a lack of porous
structures in the solid of 9. The major structural difference
between Zn-DTF 9 and Zn-TTFV 8 is that 8 assumes crys-
talline polymeric frameworks as evidenced by PXRD analysis,
whereas 9 is in the form of small clusters and amorphous (see
Figure S10 in Supporting Information File 1 for the detailed
PXRD data of 9). It is therefore reasonable to propose that the
microporosity in 8 is directly related to the coordination
polymer structure.
Finally, the thermal stability of coordination polymer 8 and
Zn-DTF 9 were evaluated by differential scanning calorimetric
(DSC) analysis, and detailed DSC traces are illustrated in
Figure 6. The DSC data of Zn-TTFV 8 (Figure 6A) manifested
very good thermal stability up to 400 °C, without any signifi-
cant melting or decomposition except a slight phase transition at
272 °C. Comparatively, the DSC trace of carboxyl-TTFV ligand
6 showed a distinctive melting process at 317 °C, which was
immediately followed by a prominent sharp exothermic peak at
326 °C (Figure 6B). The exothermic process is possibly due to a
chemical reaction(s); however, the exact reactivity awaits
further investigation to clearly elucidate. Zn-DTF 9 gave a
moderate endothermic peak at 152 °C and a significant
exothermic peak at 358 °C (Figure 6C). For carboxyl-DTF 7, a
notable melting point was observed at 199 °C, and the melting
was followed by certain exothermic processes in the range of
200 to 285 °C. The DSC results indicated that the formation of
Zn-TTFV coordinate polymer could give rise to considerably
improved thermal stability, a property particularly beneficial for
practical device and material applications.
ConclusionIn summary, we have synthesized carboxylated diphenyl-TTFV
6 and phenyl-DTF 7 as redox-active ligands to complex with
Zn(II) ions. The electronic and electrochemical properties of the
TTFV and DTF compounds were found to be in line with other
related TTFV and DTF derivatives. Of great interest is that the
complexes with Zn(II) ions retain the redox activity and electro-
chemical reactivity of their TTFV and DTF ligands in the solid
state. Another significant added value is the thermal robustness
of the Zn-TTFV coordination polymer. Collectively, the good
electrochemical and thermal properties point to a promising
prospect for them to be further developed into practically useful
organic–inorganic hybrid materials through the coordination
polymer approach. The Zn-TTFV coordination polymer was
also found to be crystalline in nature. At this stage, meaningful
single-crystal diffraction data has not yet been successfully
determined. Without such data clear understanding of the
detailed solid-state structural properties cannot be established.
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Our future work is moving towards tuning the side groups of
the TTFV ligand to produce Zn complexes with better crys-
tallinity. Finally, the Zn-TTFV coordination polymer exhibited
significant microporosity and surface area. Overall, our current
studies have cast a light on the fundamental redox and solid-
state properties of the class of TTFV-based organic–inorganic
hybrid materials, and the findings disclosed in this article
should offer useful guidance to further material design and
development.
ExperimentalChemicals were purchased from commercial suppliers and used
directly without purification. All reactions were conducted in
standard, dry glassware and under an inert atmosphere of
nitrogen unless otherwise noted. Evaporation and concentration
were carried out with a water-aspirator. Flash column chroma-
tography was performed with silica gel 60 (240-400 mesh).
Thin-layer chromatography (TLC) was carried out with silica
gel F254 covered on plastic sheets and visualized by UV light.
Melting points were measured on a SRS OptiMelt melting point
apparatus. 1H and 13C NMR spectra were measured on a Bruker
Avance III 300 MHz multinuclear spectrometer. Chemical
shifts (δ) are reported in ppm downfield relative to the signal of
the internal reference SiMe4. Coupling constants (J) are given
in Hz. Infrared spectra (IR) were recorded on a Bruker Alfa
spectrometer. HRMS analyses were performed on an Agilent
6230 TOF LC/MS instrument using an APPI ionizer. UV–vis
absorption spectra were measured on a Cary 6000i spectropho-
tometer. Cyclic voltammetric analyses were carried out in a
standard three-electrode setup controlled by a BASi epsilon
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